This invention relates to a method of controlling the feed of fuel to a two-shaft gas turbine engine, particularly during a start phase of the engine operation, and specifically to a fuel feed control system constructed to perform the same method.
Generally two-shaft gas turbine engines are operated by the employment of closed-loop control (feedback control) of fuel feed so that the rate of rotation of the power turbine during steady state operation may be kept in agreement with a predetermined target value. More particularly, the rate of fuel feed is regulated according to a control signal which may be fundamentally proportional to the magnitude of deviation of actual rate of rotation of the power turbine from the target value or may comprise a component fundamentally proportional to the deviation and another component produced by an integration of the deviation with respect to time. During a start phase of the engine operation, however, there arises a problem that, since the magnitude of the aforementioned deviation is too large during this phase, the rate of fuel feed under closed-loop control is increased by excessively large values. This problem occurs whether the control signal is a proportional signal or a proportional and integral signal, resulting and typically results in continuation of the feed of excessively large amounts of fuel.
When starting a gas turbine engine, it is necessary to maintain a specific air-to-fuel ratio which is optimum to ignition, so that fuel should be fed at a feed rate appropriate to realize the optimum air-to-fuel ratio, that is, at a rate well balanced with the rate of air intake which depends on the rate of rotation of the compressor turbine driven by a starter motor.
In view of this necessity and the above explained problem, at starting and during a certain time period immediately after starting it has been considered unsuitable to control the rate of fuel feed by a closed-loop control method wherein deviations of actual speeds of the power turbine are corrected forcibly by varying the rate of fuel feed. Conventionally, therefore, closed-loop control of fuel feed is commenced when the rate of rotation of the power turbine rises to a suitable level a certain length of time after starting of the engine. During this time period, fuel is fed substantially at a constant feed rate which is determined on the basis of experimental results to realize an air-to-fuel ratio optimum to ignition. However, this method suffers a disadvantage that the start phase of the engine operation consumes an undesirably long period of time. This extension of the start phase occurs because a desired extent of increase in the power turbine speed is awaited without taking any particular accelerating measure and without varying the fuel feed rate determined so as to be optimum to ignition, i.e. a relatively low rate of fuel feed. This disadvantage becomes very serious particularly when the gas turbine engine is used as a prime mover in an electric-power generating system for emergency service and accordingly is required to have as short as possible a starting time.